Chromatography is a procedure used for analyzing a sample to identify one or more of its constituents. The procedure for analysis involves passing a sample of the material to be analyzed through a body of material and detecting the relative separation of various sample constituents. One particular type of chromatograph is an ion chromatograph. In this device, a precise volume of sample is passed through a concentrator which traps the selected ions. After the sample has passed through, an eluent is passed through the concentrator to elute the ions and deliver them to the chromatograph for analysis.
Chromatographic analysis is a batch type operation where a sample is collected, injected and analyzed, the sample being of controlled volume to assure repeatability from sample to sample. This is necessary for example to determine changes in a process fluid which require adjustments in the process parameters. If samples of different volume are injected, and the analysis results compared, it is likely that the results will not be accurate which could lead to problems in determining the proper process parameters.
One method for providing a constant volume sample is to use a metering pump, such as a gear pump. The pump is operated for a given time period, based on the pumps' expected flow rate, to provide repeatable samples to the chromatograph. However, variations result because of differences in the sample pump suction pressure from sample to sample and also due to the presence of solids in the sample which may restrict or block flow.
In U.S. Pat. No. 3,975,946, a constant volume sampling device is described which utilizes at least two pump means, each of which is operable to supply a finite volume of carrier liquid at a flow rate determined by the rate of an input drive to the pump means. The system described is complex to assemble and operate, and additionally fails to address the problems encountered due to differences in suction pump pressures and/or with solids containing samples.